1. Field of the Invention
The invention relates to a method for determining wavefront aberrations for the characterization of imaging characteristics of an optical imaging system. The preferred field of application is the measurement of projection objectives for microlithography.
2. Description of the Related Art
Microlithographic projection exposure systems are used for producing semiconductor components and other finely structured components. In this case, a pattern of a mask or of a reticle is imaged with the aid of a projection objective on a substrate to which a light-sensitive layer has been applied. The finer the structures that are to be imaged, the greater the extent to which the quality of the projection that is produced is governed and limited by imaging errors in the optical imaging systems that are used. By way of example, these imaging errors influence the imaged line widths and the image position of the imaged structures.
The imaging characteristics are normally characterized on the basis of wavefront aberrations, in order to obtain a qualitative measure for the discrepancy between the actual image and the ideal image. Determining wavefront aberrations is a critical step in the production process of optical imaging systems, in order to make it possible to produce systems with minimal imaging errors, by means of appropriate adjustment. Since the imaging quality of optical high-performance systems is also critically dependent on environmental influences such as the temperature, pressure, mechanical loads and the like, monitoring of the imaging quality, possibly as well as aberration control by manipulations to the imaging system, are also essential at the location of use at the customer's premises. Reliable, sufficiently accurate measurement methods must be available for this purpose, to allow rapid measurement of the projection objectives in situ, that is to say in the installed space in a wafer stepper or wafer scanner.
The characterization of imaging errors by wavefront aberrations is based on the idea that, when an object in the form of a point is imaged by an ideal lens, the spherical wave which originates from the object continues on the image side of the lens as a spherical wave to the image, which is in the form of a point and which is located at the center of the spherical wave on the image side. In the case of an actual lens with aberrations, the shape of the wavefront on the image side is not a spherical shape, so that the image-side light beams are not combined in an image in the form of a point, but in a fuzzy image. In order to allow a quantitative description of the imaging errors that are produced, that wavefront which intersects the exit pupil of the imaging system on the optical axis is normally considered. The distance (in nanometres) between the actual wavefront and the ideal wavefront is referred to as the wavefront aberration. The wavefront aberration function in general has a complicated form. This function is normally described as the sum of standard functions Zi. Various groups of functions of Zi can be used for the purpose of aberration characterization. The so-called “Zernike polynomials” are normally used in the field of microlithography. The Zernike polynomials or corresponding Zernike coefficients can be derived or extracted from different measurement methods.
Various methods are available for determining the wavefront aberration at the location of use, in which case a distinction can be drawn between direct and indirect methods for determining the current status of an objective.
In one known direct method, which is referred to in the following text as the LITEL method, local tilting of the wavefront is converted with the aid of a specially constructed reticle into distortion in the imaging plane. This is then measured using a standard box-in-box method, and the wavefront is reconstructed by calculation from this. The accuracy of the method is sufficient for most applications. However, the analysis time is in the range of several hours.
Measurements with different NA and exposure settings (multiple illumination settings, MIS) are carried out in each case for the available indirect methods. In this case, a distinction can be drawn between aerial imaging measurements and resist profile measurements. One resist-based measurement technique is the so-called aberration ring test (ART) (see, for example the article “Impact of high order aberrations on the performance of the aberration monitor” by P. Dirksen, C. Juffermans, A. Engelen, P. De Bisschop, H. Muellerke, Proc. SPIE 4000 (2000), pages 9 et seq. or “Application of the aberration ring test (ARTEMIS™) to determine lens quality and predict its lithographic performance”, by M. Moers, H. van der Laan, M. Zellerrath, W. de Boeij, N. Beaudry, K. D. Cummings, A. van Zwolt, A. Becht and R. Willekers, lecture to the SPIE, the 26th Annual International Symposium on Microlithography, 25 Feb. to 2 Mar. 2001,Santa Clara, Calif.). In the aberration ring test, an annual object is imaged in the imaging plane. The ring diameter and ring shape deformations which can be measured on the imaged object for a focus series can be analyzed in the form of Fourier components, with each Fourier component corresponding to a specific class of aberrations (lumped aberrations), for example spherical, coma, astigmatism and trefoil distortion. It is assumed that there is an essentially linear relationship between the Fourier components which can be determined in this way and the wavefront aberrations which can be described by Zernike coefficients, so that individual Zernike coefficients can be extracted on the basis of a suitable model from lumped aberrations. This method has the disadvantage that the accuracy is dependent on the quality of the model which is used in the simulation. A comparison with aberration data which was obtained with the aid of high-precision direct interferometric measurements of wavefront aberrations (see DE 101 09 929) shows, however, that the present model is suitable only to a limited extent. Apart from this, the method becomes less stable the greater the number of orders of Zernike coefficients that are intended to be separated.